The myogenic differentiation 1 (Myodl) gene encodes a basic helix-loop-helix transcription factor that is a master regulator of skeletal muscle lineage determination during embryonic development. The post-natal function of Myodl is not well understood but a deficit in the specific tension of Myodl (-/-) mice, in addition to their impaired regenerative ability, intimate a significant role in adult skeletal muscle. We recently identified the Myodl transcript in adult skeletal muscle as having a robust circadian pattern of expression and this pattern of expression was absent in muscle of homozygous Clock mutant mice, suggesting Myodl is a clockcontrolled gene. The long-range goal of the project is to understand the circadian function of Myodl in adult skeletal muscle. As a first step toward this goal, the objective of this proposal will be to determine if the coreclock transcription factors Clock and Email are directly involved in the regulation of Myodl transcription. The primary hypothesis of this application is that Myodl is a direct transcriptional target of the CLOCK:BMAL1 heterodimer. The hypothesis will be tested by pursuing the following three specific aims: 1) use the Myodl DRRIoxP and CEIoxP mice to determine the role of the distal regulatory region (DRR) and the core enhancer (CE) of the Myodl promoter in the circadian regulation of Myodl transcription, 2) determine the ability of core-clock transcription factors CLOCK and BMAL1 to transactivate Myodl reporter genes and 3) determine if CLOCK and BMAL1 are bound to the endogenous Myodl promoter by chromatin immunoprecipitation (ChIP) assay. The discovery of Myodl as a circadianly expressed gene is an exciting discovery because it provides a completely novel paradigm for thinking about the post-natal function of Myodl and establishes a potential link between master regulators of circadian rhythms and tissue-specific expression. A better understanding of how Myodl transcription is regulated in the adult is expected to provide a foundation for understanding the mechanisms underlying the altered expression of Myodl in such diseases as muscular dystrophy and sarcopenia.